Stabilizers to inhibit the polymerization of substituted cyclotetrasiloxane

ABSTRACT

The present invention is; (a) a process for stabilizing a cyclotetrasiloxane, such as 1,3,5,7-tetramethylcyclotetrasiloxane, against polymerization used in a chemical vapor deposition process for silicon oxides in electronic material fabrication comprising providing an effective amount of a free radical scavenger polymerization inhibitor to such cyclotetrasiloxane; and (b) a composition of a cyclotetrasiloxane, such as 1,3,5,7-tetramethylcyclotetrasiloxane, stabilized against polymerization used in a chemical vapor deposition process as a precursor for silicon oxides in electronic material fabrication, comprising; such cyclotetrasiloxane and a free radical scavenger polymerization inhibitor.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a continuation-in-part of U.S. application Ser. No.10/029,892, now U.S. Pat. No. 6,858,697, filed Dec. 21, 2001, which isincorporated herein by reference in its entirety.

BACKGROUND OF THE INVENTION

Silicon dioxide films have been used for some time in the fabrication ofintegrated circuits (IC) for semiconductor device manufacturing. Thereare many examples of the preparation of such thin films of SiO₂ in theopen and patent literature. See, for example, the publications of theSchumacher Group, Air Products and Chemicals, Inc., e.g. User's GuideFor: Glass Deposition with TEOS¹, and Extrema® TEOS (TetraethylOrthosilicate) Product Data Sheet². See also, Modeling of Low-PressureDeposition of SiO₂ by Decomposition of TEOS³, and The Deposition ofSilicon Dioxide Films at Reduced Pressure⁴. There are numerous journalarticles that review various CVD techniques for the deposition of SiO₂and the properties of thin films deposited using such techniques⁵⁻⁹.

Early SiO₂ films were deposited by CVD oxidation of silane (SiH₄). Newsource materials were needed in order to maintain good step coverage assub-micron patterned electronic devices were developed. Films depositedfrom tetraethylorthosilcate (TEOS) show superior step coverageproperties compared to SiH₄ ⁷. TEOS is considered an industry standardsource for the CVD preparation of SiO₂. TEOS is a volatile liquid,providing for efficient vapor delivery and general ease of handling. Itis nonpyrophoric, and therefore, provides a significant safety advantageover silane. It produces dielectric films with excellent electrical andmechanical properties suitable for many device manufacturingapplications.

The chemical 1,3,5,7-Tetramethylcyclotetrasiloxane (such as TOMCATS®siloxane available from Schumacher of Carlsbad, Calif.) is underdevelopment as a new source material for the CVD preparation of SiO₂glass¹⁰⁻¹¹. TOMCATS type siloxane is a high purity volatile liquidprecursor chemical that is specifically designed to satisfy the criticaldemands of the semiconductor device manufacturing industry. Like TEOS,TOMCATS type siloxane can be used for the chemical vapor deposition ofglasses and doped glasses for various dielectric film applications suchas trench fill, interlevel dielectric, gate and thick oxide². Itprovides similar safety advantages because of its non-pyrophoric andnoncorrosive nature. The normal boiling points of TOMCATS type siloxaneand TEOS are 135° C. and 168° C., respectively. The higher volatility ofTOMCATS type siloxane allows it to be delivered at lower temperature orwith higher efficiency at comparable temperature. Its deposition rate is10 times that of TEOS at 600° C., with a deposition efficiency 3 timesthat of TEOS². It is superior to silane and similar to TEOS in theconformality and step coverage of the resulting films¹¹⁻¹².

In general, SiO₂ films deposited from TOMCATS type siloxane exhibitexcellent mechanical and electrical properties. The films are dense withlow carbon content and refractive index values comparable to thermaloxide. TOMCATS type siloxane is effective for low-pressure chemicalvapor deposition (LPCVD) and as a liquid injection source for plasmaenhanced chemical vapor deposition (PECVD). The later method utilizesplasmas rather than thermal energy to promote chemical reactions.TOMCATS type siloxane PECVD is typically run at lower temperature thanLPCVD (400° C. vs. 500–600° C.).

Despite these advantages, TOMCATS type siloxane has experienced limitedacceptance as a CVD source for the manufacturing of semiconductordevices. One disadvantage of TOMCATS type siloxane is its instabilitywith respect to polymerization¹³ when exposed to certain chemicals orprocess conditions. This results in a lower volatility liquid or gelthat creates CVD process handling issues. TOMCATS type siloxanepolymerization is catalyzed by acid, base or free radicals.

Prolonged heating of TOMCATS type siloxane (Example 1) has also beenshown experimentally in the present invention to promote polymerization.The degree of polymerization can be very minor, accounting for onlyseveral tenths of a percent. Under more severe conditions of prolongedexposure to elevated temperature or to certain acids or bases,substantial polymerization will occur, resulting in a highly viscousliquid or gel containing over 10% by weight of oligomeric or polymericmaterial.

Several references in the prior art relate to the stabilization ofsiloxane. Hirabayashi et al.¹⁴ teach the use of a triazine or sulfide“control agent” to stabilize a mixture comprising an aliphaticunsaturated group, containing an organopolysiloxane compound, such asTOMCATS type siloxane, and a platinum group catalyst. Those inventorsteach the use of the triazine or sulfide agent to give a mixture that isstable and resistant to premature gelation at room temperature and thusproviding extended storage stability.

Lutz et al. ¹⁵ disclose the use of di- and trihydrocarbylphosphineswhich act as curing inhibitors for compositions comprising: (1) alkenylradicals; (2) compounds containing silicon-bonded hydrogen atoms (e.g.,TOMCATS type siloxane); and (3) a platinum group metal catalyst. Lutz etal. claim that the inhibitor functions by complexing with the platinumcatalyst rendering it inactive for subsequent curing.

In a similar patent, Chalk¹⁶ teaches the use of acrylonitrile typecompounds that reduce the activity of the platinum catalyst deterringthe copolymerization of various mixtures of polysiloxanes.

Berger et al. ¹⁷ propose the use of an ethylenically unsaturatedisocyanurate which functions in a like manner to deactivate the Ptcatalyst rendering a curable organopolysiloxane composition stable topremature gelation.

Endo et al. ¹⁸ teach the stabilization of cyclosiloxanes, such asTOMCATS type siloxane through the use of 1 to 20 weight % ofpolymethylpolysiloxanes, such as 1,1,1,3,5,5,5-heptamethyltrisiloxane.

The patent references cited all teach the use of various agents that inone manner or another inhibit the polymerization or co-polymerization ofpolysiloxanes for various applications in the silicon rubber industry.None of them specify or suggest applications as polymerizationinhibitors for CVD sources in the semiconductor device manufacturingindustry.

BRIEF SUMMARY OF THE INVENTION

The present invention is a process for stabilizing a substitutedcyclotetrasiloxane against polymerization used in a chemical vapordeposition process for silicon oxides in electronic material fabricationcomprising providing an effective amount of a free radical scavengerpolymerization inhibitor to a substituted cyclotetrasiloxane having thefollowing formula:

where R¹⁻⁷ are individually selected from the group consisting ofhydrogen, a normal, branched or cyclic C₁₋₁₀ alkyl group, and a C₁₋₄alkoxy group.

The present invention is also a composition of substitutedcyclotetrasiloxane stabilized against polymerization used in a chemicalvapor deposition process as a precursor for silicon oxides in electronicmaterial fabrication, comprising; (a) a substituted cyclotetrasiloxanehaving the following formula:

where R¹⁻⁷ are individually selected from the group consisting ofhydrogen, a normal, branched or cyclic C₁₋₁₀ alkyl group, and a C₁₋₄alkoxy group, and (b) a free radical scavenger.

DETAILED DESCRIPTION OF THE INVENTION

The chemical 1,3,5,7-tetramethylcyclotetrasiloxane (such as TOMCATS®siloxane available from Schumacher of Carlsbad, Calif.) is used as aprecursor for the chemical vapor deposition (CVD) of SiO₂ forsemiconductor device manufacturing. TOMCATS type siloxane is currentlyunder evaluation by semiconductor device manufacturers for use as a CVDprecursor for SiO₂ because of its ability to form high quality filmswith excellent electronic and mechanical properties. TOMCATS typesiloxane is known to polymerize when subjected to extended periods ofheating or upon exposure to certain chemicals. In this invention wedisclose the use of various free radical scavengers that inhibit thepolymerization of TOMCATS type siloxane. The low concentration of theadditive does not significantly impact the overall product purity, noris it anticipated to have a negative impact on the critical propertiesof the resulting films produced by CVD.

Therefore, an object of the present invention is to eliminate or inhibitthe polymerization of TOMCATS type siloxane under typical CVD processconditions. These TOMCATS type siloxanes include substitutedcyclotetrasiloxanes of the formula:

where R¹⁻⁷ are individually selected from the group consisting ofhydrogen, a normal, branched or cyclic C₁₋₁₀ alkyl group, and a C₁₋₄alkoxy group.

This is done through the use of additives that inhibit thepolymerization of TOMCATS type siloxane under conditions that wouldnormally favor polymerization. The present invention demonstrates thatcertain additives are effective at inhibiting polymerization, such asfree radical scavengers. TOMCATS type siloxanes are sensitive to oxygen,carbon dioxide and nitrogen trifluoride (NF₃) at elevated temperatures.TOMCATS type siloxanes react with oxygen forming oligomeric andpolymeric species at temperatures equal to or greater than 60° C. Thisis significant because oxygen, carbon dioxide and nitrogen trifluorideare commonly used in the manufacture of semiconductor devices, such asthe oxidizing gas in plasma enhanced chemical vapor deposition (PECVD)processes for the deposition of SiO₂ films from TOMCATS type siloxane.These scavengers work by deterring chemical reactions that proceed by afree-radical reaction pathway. The free radical scavengers contemplatedas O₂-, CO₂- and/or NF₃-stabilizers are 2,6-di-tert-butyl-4-methylphenol (or BHT for butylhydroxytoluene),2,2,6,6-tetramethyl-1-piperidinyloxy (TEMPO),2-tert-butyl-4-hydroxyanisole, 3-tert-butyl-4-hydroxyanisole, propylester 3,4,5-trihydroxy-benzoic acid,2-(1,1-dimethylethyl)-1,4-benzenediol, diphenylpicrylhydrazyl,4-tert-butylcatechol, N-methylaniline, p-methoxydiphenylamine,diphenylamine, N,N′-diphenyl-p-phenylenediamine, p-hydroxydiphenylamine,phenol, octadecyl-3-(3,5-di-tert-butyl-4-hydroxyphenyl) propionate,tetrakis (methylene (3,5-di-tert-butyl) -4-hydroxy-hydrocinnamate)methane, phenothiazines, alkylamidonoisoureas, thiodiethylene bis(3,5,-di-tert-butyl-4-hydroxy-hydrocinnamate, 1,2,-bis(3,5-di-tert-butyl-4-hydroxyhydrocinnamoyl) hydrazine, tris(2-methyl-4-hydroxy-5-tert-butylphenyl) butane, cyclic neopentanetetraylbis (octadecyl phosphite), 4,4′-thiobis (6-tert-butyl-m-cresol),2,2′-methylenebis (6-tert-butyl-p-cresol), oxalyl bis(benzylidenehydrazide) and naturally occurring antioxidants such as rawseed oils, wheat germ oil, tocopherols and gums.

Preferably, the free radical scavenger is provided in an amount of10–1000 ppm (wt.); more preferably an amount of of 50–500 ppm (wt.);most preferably, an amount of 50–250 ppm (wt.); optimally, an amount ofof 100–200 ppm (wt.).

To attain the object of the present invention, to eliminate or inhibitthe polymerization of TOMCATS type siloxane under typical CVD processconditions, a standard laboratory test was established with the intentof accelerating the normal polymerization process. The accelerated agingtest is meant to simulate the normal course of gradual polymerizationthat would typically occur over a more protracted period of time. Thistest, which consists of exposing a sealed quartz ampoule of TOMCATS typesiloxane to elevated temperature for 24 hours, is referred to in thepresent document as the “accelerated aging test”. These conditions areunderstood to be considerably more severe than TOMCATS type siloxanewould be subjected to in a typical CVD process. In a typical acceleratedaging test, the ampoule is loaded with approximately 5.0 ml of TOMCATStype siloxane and, except for “control experiments”, a free radicalscavenger to inhibit polymerization. The TOMCATS type siloxane/additivemixture is cooled in a liquid nitrogen bath. Then, the atmosphere abovethe TOMCATS type siloxane is evacuated for 5 minutes. The neck of thequartz ampoule is subsequently sealed using a hydrogen/oxygen torch. Thesealed ampoule is placed in an oven and held at 120° C. for 5 days. Theampoule is removed and allowed to cool to room temperature. Its contentsare analyzed by gas chromatograph (GC) to measure the degree ofpolymerization.

The degree of polymerization is measured quantitatively by GC. Thistechnique is very sensitive to detecting the onset of polymerization asevidenced by the formation of higher molecular weight species withlonger retention times than the parent TOMCATS type siloxane peak.TOMCATS type siloxane samples that are determined to be of “highviscosity” by visual inspection are not routinely run on the GC. Theoligomeric or polymeric siloxanes tend to irreversibly contaminate thestationary phase of the GC column due to their low solubility and lowvolatility. Such samples are qualitatively described in the presentinvention to have greater than 10 wt. % polymer, consistent withprevious observations.

The polymerization of cyclical polysiloxanes has been determined to becatalyzed by free radicals. Laboratory observations suggest that thepolymerization of TOMCATS type siloxane is particularly sensitive toexposure to oxygen or nitrogen trifluoride, both of which the siloxaneis exposed to in use in semiconductor manufacture. The additivesdescribed in this invention form solutions with TOMCATS type siloxane atthe tested concentrations. In addition, these additives are notanticipated to have a detrimental impact on the overall CVD process byvirtue of their concentration and their chemical and physicalcharacteristics.

In-house experiments have established that TOMCATS type siloxane issensitive to oxygen and/or nitrogen trifluoride at elevatedtemperatures. TOMCATS type siloxane reacts with oxygen and/or nitrogentrifluoride forming oligomeric and polymeric species at temperaturesequal to or greater than 60° C. This is particularly important sinceoxygen and/or nitrogen trifluoride is commonly used as the oxidizing gasin PECVD processes for the deposition of SiO₂ films from TOMCATS typesiloxane or as a cleaning gas between production runs. Data collectedfor the stability of TOMCATS type siloxanes in the presence of oxygen,carbon dioxide and nitrogen trifluoride are shown in Table1.

To address this reactivity TOMCATS type siloxane was spiked with lowlevels of chemicals which function as free radical scavengers, i.e.,antioxidants. These scavengers work by deterring chemical reactions thatproceed by a free-radical reaction pathway. The free radical scavengerinvestigated as O₂-, CO₂- and/or nitrogen trifluoride- stabilizers was2,6-di-tert-butyl-4-methyl phenol (or BHT for butylhydroxytoluene.TOMCATS type siloxane was substantially more resistant toward O₂, CO₂and/or nitrogen trifluoride when spiked with BHT. The addition of 150ppm by weight of BHT greatly reduced the sensitivity of TOMCATS typesiloxane toward O₂, CO₂ and/or nitrogen trifluoride at elevatedtemperature as shown by the series of tests run at 90° C. (Table 1).Another benefit is that BHT is free of atomic nitrogen which reportedlygives rise to undesirable basic film properties. TEMPO is also expectedto be an effective O₂, CO₂ and/or nitrogen trifluoride-stabilizer.

These tests clearly established the benefit of the use of low levels offree radical scavengers to greatly reduce or eliminate the sensitivityof TOMCATS type siloxane to O₂, CO₂ and/or nitrogen trifluoride,thereby, reducing the likelihood of plugging problems occurring by theO₂, CO₂ and/or nitrogen trifluoride promoted polymerization of TOMCATStype siloxane. The scavengers/antioxidants contemplated for this utilityinclude: 2,6-di-tert-butyl-4-methyl phenol,2,2,6,6-tetramethyl-1-piperidinyloxy, 2-tert-butyl-4-hydroxyanisole,3-tert-butyl-4-hydroxyanisole, propyl ester 3,4,5-trihydroxy-benzoicacid, 2-(1,1-dimethylethyl) -1,4-benzenediol, diphenylpicrylhydrazyl,4-tert-butylcatechol, N-methylaniline, p-methoxydiphenylamine,diphenylamine, N,N′-diphenyl-p-phenylenediamine, p-hydroxydiphenylamine,phenol, octadecyl-3-(3,5-di-tert -butyl-4-hydroxyphenyl) propionate,tetrakis (methylene (3,5-di-tert-butyl)-4-hydroxy-hydrocinnamate)methane, phenothiazines, alkylamidonoisoureas, thiodiethylene bis(3,5,-di-tert-butyl-4-hydroxy-hydrocinnamate, 1,2,-bis (3,5-di-tert-butyl-4-hydroxyhydrocinnamoyl) hydrazine, tris(2-methyl-4-hydroxy-5-tert-butylphenyl) butane, cyclic neopentanetetraylbis (octadecyl phosphite), 4,4′-thiobis (6-tert-butyl-m-cresol),2,2′-methylenebis (6-tert-butyl-p-cresol), oxalyl bis(benzylidenehydrazide) and mixtures thereof. Naturally occurringantioxidants can also be used such as raw seed oils, wheat germ oilstocopherols and gums.

The polymerization of TOMCATS type siloxanes is known to be catalyzed byfree radicals. The present invention demonstrates that certain freeradical scavengers are effective additives for inhibiting thepolymerization of TOMCATS type siloxanes, such as2,6-di-tert-butyl-4-methyl phenol, also known as butylhydroxytoluene(BHT).

To attain the object of the present invention, to eliminate or inhibitthe polymerization of TOMCATS type siloxane under typical CVD processconditions, laboratory experiments were run with the intent ofsimulating conditions that TOMCATS type siloxane would be subject to ina typical CVD process. The effectiveness of these inhibitors was gaugedby comparing the stability of neat TOMCATS type siloxane (i.e., nopolymerization inhibitor) with that of TOMCATS type siloxane stabilizedwith free radical scavengers such as BHT. These stability tests werecarried out at 90° C. in the absence of contaminants (under vacuum), andin presence of contaminants, in which TOMCATS type siloxane wasintentionally exposed to controlled amounts of selected gases such asO₂, CO₂ and nitrogen trifluoride. All three of these gases are typicallyused at some point in the processing or maintenance for the chemicalvapor deposition of SiO₂ from TOMCATS type siloxane precursor. Oxygenand NF₃ are known sources of free radicals. TOMCATS type siloxane isoften diluted with O₂ and/or CO₂ during a typical PECVD process.Nitrogen trifluoride is commonly used in the chamber-cleaning step ofsuch processes.

EXAMPLE 1 Polymerization Under Vacuum Conditions Stability of TOMCATSType Siloxane, With and Without BHT

Six quartz ampoules with a nominal volume of 80–90 ml were used for thistest. These ampoules will be referred to in the present example as 1A,1B, 1C, 1D, 1E and 1F. These ampoules were prepared by rinsing twicewith distilled water, twice with reagent grade acetone, then placed intoa drying oven at 175° C. for 16–18 hours. The dry ampoules were removedfrom the oven and used while still warm. Approximately 5.0 ml ofadditive free TOMCATS type siloxane was loaded into ampoules 1A, 1B, 1Cand 1D. A similar amount of TOMCATS type siloxane containing 150 ppm (byweight) BHT was loaded into ampoules 1E and 1F. Teflon valves wereattached to the open end of the ampoules. The end of ampoule 1A wasimmersed in a liquid nitrogen bath to cause any vaporized TOMCATS typesiloxane to condense. The air was evacuated from the headspace of theampoule by subjecting it to vacuum for 5 minutes. The ampoule was sealedat the neck using a hydrogen/oxygen torch. The remaining 5 ampoules(1B–1F) were sealed in a similar fashion. Sealed ampoules 1C, 1D, 1E and1F were placed in a nitrogen-purged oven, and subsequently held at aconstant temperature of 90° C. for 24 hours. Ampoules 1A and 1B werekept at room temperature and served as control samples. After 24 hoursthe heated ampoules were removed from the oven and allowed to cool toroom temperature.

GC analysis showed no significant polymerization for the control samples(1A, 1B) relative to the lot material. The heated samples with noadditive (1C, 1D) showed an average polymerization of 0.136%. The heatedsamples with 150 ppm BHT had an average polymerization of 0.079%.Results are summarized in Table 1.

EXAMPLE 2 Sensitivity to Carbon Dioxide Exposure of TOMCATS TypeSiloxane to 0.50 Weight % Carbon Dioxide

Four quartz ampoules (2A, 2B, 2C and 2D) were cleaned and dried asdescribed in Example 1.Approximately 5.0 g of TOMCATS type siloxanecontaining no additive was loaded into ampoules 2A and 2B. A similaramount of TOMCATS type siloxane spiked with 150 ppm by weight of BHT wasloaded into ampoules 2C and 2D. Each of the 4 Ampoules was equipped witha quartz side-arm extension, capped with a septum. Ampoule 2A was cooledto liquid nitrogen temperature and evacuated to remove the air in theheadspace. The ampoule was isolated from the vacuum and 19 sccm ofgaseous carbon dioxide was injected via a syringe through the septum capon the side arm. The ampoule, still under sub-ambient pressure, wassealed using a torch as described in Example 1. The remaining 3 ampoules(2B, 2C and 2D) were prepared and sealed in the same manner. All foursealed ampoules were heated for 24 hours at 90° C. as described inExample 1. TOMCATS type siloxane without additive showed an averagepolymerization of 0.216%. The same chemical with 150 ppm of BHT additiveshowed an average polymerization of 0.028%. Results are summarized inTable 1.

EXAMPLE 3 Sensitivity to Oxygen Exposure of TOMCATS Type Siloxane to0.50 Weight % Oxygen

Four quartz ampoules (3A, 3B, 3C and 3D) were cleaned and dried asdescribed in Example 1.Approximately 5.0 g of TOMCATS type siloxanecontaining no additive was loaded into ampoules 3A and 3B. A similaramount of TOMCATS type siloxane spiked with 150 ppm by weight of BHT wasloaded into ampoules 3C and 3D. Each of the 4 ampoules was equipped witha quartz side-arm extension, capped with a septum. Ampoule 3A was cooledto liquid nitrogen temperature and evacuated to remove the air in theheadspace. The ampoule was isolated from the vacuum and 19 sccm ofoxygen was injected via a syringe through the septum cap on the sidearm. The ampoule, still under sub-ambient pressure, was sealed using atorch as described in Example 1. The remaining 3 ampoules (3B, 3C and3D) were prepared and sealed in the same manner. All four sealedampoules were heated for 24 hours at 90° C. as described in Example 1.TOMCATS type siloxane without additive showed an average polymerizationof 6.462%. The same chemical with 150 ppm of BHT additive showed anaverage polymerization of 0.031%. Results are summarized in Table 1.

EXAMPLE 4 Sensitivity to Nitrogen Trifluoride Exposure of TOMCATS TypeSiloxane Without BHT to Nitrogen Trifluoride

Compatibility tests were carried to evaluate the effectiveness of freeradical scavengers, such as BHT, to inhibit the nitrogen trifluoridepromoted polymerization of TOMCATS type siloxane. Because of thepotential reactivity of NF₃ And the corrosive nature of possiblybyproducts, these compatibility tests were carried out in a 300 ccstainless steel Parr Reactor.

49.956 g of TOMCATS type siloxane was loaded into the 300 cc reactor.This sample of TOMCATS type siloxane did not have BHT, but did have 125ppm by weight 2,4-pentanedione. The 2,4-pentanedione was developed as anearlier additive to stabilize TOMCATS type siloxane. The gas in thereactor headspace was evacuated. NF₃ was expanded into the headspacesuch that its final concentration was 636 ppm by weight (0.0636 weight%). The reactor temperature was raised to 100° C. and held for 24 hours.After the specified time, the NF₃ was removed by pumping out thereactor. The reactor was opened. The TOMCATS type siloxane hadcompletely gelled. There was no residual liquid in the reactor.

Samples that are very viscous or gelled, such as the one described inthe present example, are indicative of a high degree of polymerizationfor TOMCATS type siloxane. These samples are not amenable to analysis byGC due to their insolubility in common organic solvents. Such samplesare assigned a degree of polymerization of “>10 weight %” for thepurpose of this document.

EXAMPLE 5 Sensitivity to Nitrogen Trifluoride Exposure of TOMCATS TypeSiloxane With 150 ppm BHT to Nitrogen Trifluoride

49.863 g of TOMCATS type siloxane was loaded into the 300 cc reactor.This sample of TOMCATS type siloxane had been previously spiked with 150ppm by weight of BHT. The gas in the reactor headspace was evacuated.NF₃ was expanded into the headspace such that its final concentrationwas 631 ppm by weight (0.0631 weight %). The reactor temperature wasraised to 100° C. and held for 24 hours. After the specified time, theNF₃ was removed by pumping out the reactor. The reactor was opened and45.631 g of clear colorless liquid was recovered. The loss in weight wasprobably due to pumping on the reactor at the end of the experiment toremove the NF₃. The liquid was transferred to a polyethylene bottle. Asample was analyzed by GC, establishing that the purity of TOMCATS typesiloxane stayed the same at 99.95% before and after analysis. Nopolymerization was detected.

TABLE 1 The stability of TOMCATS type siloxane with and without BHTinhibitor in the presence of various chemical sources of free radicals.% Purity of TOMCATS Spiked Average % type with Time Extent ofPolymerization siloxanes 150 @ Polymerization of duplicate Example Gasin (before ppm 90° C. after testing samples (after No. Headspace*testing) BHT? (hrs) (%) testing) 1A None 99.962 No  0   <0.005 <0.005 1BNone 99.962 No  0    <0.005 1C None 99.962 No 24     0.113 0.136 1D None99.962 No 24     0.159 1E None 99.962 Yes 24     0.084 0.079 1F None99.962 Yes 24     0.075 2A CO₂ 99.962 No 24     0.242 0.216 2B CO₂99.962 No 24     0.190 2C CO₂ 99.962 Yes 24     0.028 0.028 2D CO₂99.962 Yes 24     0.027 3A O₂ 99.962 No 24     6.482 6.462 3B O₂ 99.962No 24     6.442 3C O₂ 99.962 Yes 24     0.006 0.031 3D O₂ 99.962 Yes 24    0.057 4 NF₃ 99.93 No 24 ¥ >10.0 ‡ >10.0 5 NF₃ 99.95 Yes 24 ¥  <0.01<0.01 *All contaminant gases were spiked at 0.50 weight percent, withthe exception of NF₃ in Examples #4, and #5, that was present at 0.0636wt. % and 0.0631 wt. %, respectively. ¥ Testing temperature was 100° C.‡ No GC was run since this sample had fully gelled. This is indicativeof >10% polymerization.

The present invention has been set forth with regard to severalpreferred embodiments, but the full scope of the present inventionshould be ascertained from the claims which follow.

1. A process for stabilizing a cyclotetrasiloxane against polymerizationused in a chemical vapor deposition process for silicon oxides inelectronic material fabrication and stabilized for extended periods ofheating, comprising; providing an effective amount of a free radicalpolymerization inhibitor to said cyclotetrasiloxane having the followingformula:

where R¹⁻⁷ are individually selected from the group consisting ofhydrogen, a normal, branched or cyclic C₁₋₁₀ alkyl group, and a C₁₋₄alkoxy group, wherein said free radical scavenger is selected from thegroup consisting of: 2,6-ditert-butyl-4-methyl phenol,2,2,6,6-tetramethyl-1-piperidinyloxy, 2,6-dimethylphenol,2-tert-butyl-4-hydroxyanisole, 3-tert-butyl-4-hydroxyanisole, propylester 3,4,5-trihydroxy-benzoic acid, 2-(1,1-dimethylethyl)-1,4benzenediol, diphenylpicrylhydrazyl, 4-tert-butylcatechol,N-methylaniline, 2,6-dimethylaniline, p-methoxydiphenylamine,diphenylamine, N,N′-diphenyl-p-phenylenediamine, p-hydroxydiphenylamine,phenol, octadecyl-3-(3,5-di-tert-butyl-4 -hydroxyphenyl) propionate,tetrakis (methylene (3,5-di-tert-butyl-4-hydroxy-hydrocinnamate)methane, phenothiazines, alkylamidonoisoureas, thiodiethylene bis(3,5,-di-tert-butyl-4-hydroxy-hydrocinnamate, 1,2,-bis(3,5-di-tert-butyl-4-hydroxyhydrocinnamoyl) hydrazine, tris(2-methyl-4-hydroxy-5-tert-butylphenyl) butane, cyclic neopentanetetraylbis (octadecyl phosphite), 4,4′-thiobis (6-tert-butyl-m-cresol,2,2′-methylenebis (6-tert-butyl-p-cresol), oxalyl bis(benzylidenehydrazide) and mixtures thereof.
 2. The process of claim 1wherein said free radical scavenger is 2,6-di-tert-butyl-4-methylphenol.
 3. A process for stabilizing a cyclotetrasiloxane againstpolymerization used in a chemical vapor deposition process for siliconoxides in electronic material fabrication, comprising; providing aneffective amount of a free radical polymerization inhibitor to saidcyclotetrasiloxane having the following formula:

where R¹⁻⁷ are individually selected from the group consisting ofhydrogen, a normal, branched or cyclic C₁₋₁₀ alkyl group, and a C₁₋₄alkoxy groups wherein said free radical scavenger is provided in anamount of 10–1000 ppm (wt.).
 4. The process of claim 3 wherein said freeradical scavenger is provided in an amount of 50–500 ppm (wt.).
 5. Theprocess of claim 3 wherein said free radical scavenger is provided in anamount of 50–250 ppm (wt.).
 6. The process of claim 3 wherein said freeradical scavenger is provided in an amount of 100–200 ppm (wt.).
 7. Aprocess for stabilizing 1,3,5,7-tetramethylcyclotetrasiloxane againstpolymerization for extended periods of heating and caused by oxygen,carbon dioxide and/or nitrogen trifluoride used in a chemical vapordeposition process for silicon oxides in electronic material fabricationcomprising providing a free radical scavenger to said1,3,5,7-tetramethylcyclotetrasiloxane.
 8. The process of claim 7 whereinsaid free radical scavenger is selected from the group consisting of2,6-di-tert-butyl-4-methyl phenol, 2,2,6,6-tetramethyl-1-piperidinyloxyand mixtures thereof.
 9. A composition of1,3,5,7-tetramethylcyclotetrasiloxane stabilized against polymerizationfor extended periods of heating and used in a chemical vapor depositionprocess as a precursor for silicon oxides in electronic materialfabrication comprising 1,3,5,7-tetramethylcyclotetrasiloxane and a freeradical scavenger polymerization inhibitor.
 10. A composition of1,3,5,7-tetramethylcyclotetrasiloxane, used in a chemical vapordeposition process as a precursor for silicon oxides in electronicmaterial fabrication, and stabilized against polymerization for extendedperiods of heating, comprising (a)1,3,5,7-tetramethylcyclotetrasiloxane, (b) a free radical scavengerselected from the group consisting of 2,6-di-tert-butyl-4-methyl phenol,2,2,6,6-tetramethyl -1-piperidinyloxy, 2-tert-butyl-4-hydroxyanisole,3-tert-butyl-4-hydroxyanisole, propyl ester 3,4,5-trihydroxy-benzoicacid, 2-(1 ,1-dimethylethyl)-1,4-benzenediol, diphenylpicrylhydrazyl,4-tert -butylcatechol, N-methylaniline, p-methoxydiphenylamine,diphenylamine, N,N′-diphenyl -p-phenylenediamine,p-hydroxydiphenylamine, phenol, octadecyl-3-(3,5-di-tert-butyl-4-hydroxyphenyl) propionate, tetrakis (methylene(3,5-di-tert-butyl)-4-hydroxy-hydrocinnamate) methane, phenothiazines,alkylamidonoisoureas, thiodiethylene bis (3,5,-di-tert-butyl-4-hydroxy-hydrocinnamate, 1,2,-bis(3,5-di-tert-butyl-4-hydroxyhydrocinnamoyl) hydrazine, tris(2-methyl-4-hydroxy-5-tert-butylphenyl) butane, cyclic neopentanetetraylbis (octadecyl phosphite), 4,4′-thiobis (6-tert-butyl-m-cresol),2,2′-methylenebis (6-tert-butyl-p-cresol), oxalyl bis(benzylidenehydrazide) and mixtures thereof.